This makes rdt work again, which has been broken since603b8bc2a109978c8499b06d2556f1433306eca7. This commit made
opening a demuxer without a file (or in this case, with a filename
which can't be opened) fail, unless the demuxer actually declared
AVFMT_NOFILE.

Since 1.0.0, this function is deprecated. A new function,
CRYPTO_THREADID_set_callback is available, but if not set at all,
it uses the address of errno as thread id, which should be
sufficient for most systems.

On windows, it never was necessary to use this function even
before 1.0.0, it used the right win32 API function for this
by default.

Calling the init function will become mandatory at some later
point. By calling it, more heavy network init (such as SSL/TLS
library init) can be done once at startup, instead of implicitly
when used (which could lead to it being done a number of times).

We keep INIT_AVX (for backwards compatibility). 3arg AVX ops with
a memory arg can only have it in src2, whereas SSE emulation of
3arg prefers to have it in src1 (i.e. the mov). So, if the op is
symmetric and the wrong one is memory, swap them.

This function used to set codec_type. With the current fallback
implementation based on avcodec_get_context_defaults3, codec_type
won't be set to the value passed in, but will be set to
AVMEDIA_TYPE_UNKNOWN. Legacy callers of this function might expect
this field to be set to the value passed in.

If attribute_deprecated is used in an enum declaration, it
should follow the 'enum' keyword, otherwise it's ignored
silently. This is the only case of attribute_deprecated for
enum declarations currently.

Currently, this attribute_deprecated doesn't have any effect.
If moved to the right place, it emits a warning every single
time avcodec.h is included, like this: